1. Field of the Invention
The present invention relates to a discharge-pumped gas laser with preionizers, and more particularly to a discharge-pumped gas laser such as a discharge-pumped excimer laser which has a preionizing circuit substantially independent of a main discharge circuit.
2. Description of the Prior Art
Some discharge-pumped gas lasers, e.g., discharge-pumped rare-gas-halide excimer lasers, include preionizers for preionizing the laser gas to stabilize a discharge between main discharge electrodes. Particularly, preionization of the laser gas for a stable discharge is indispensable for excimer lasers. Laser gas preionization is discussed in the literature Development of Excimer Lasers and their Application Technologies and Examples, written by Shuntaro Watanabe and published by Applied Technology Publishing, pages 15 through 30, for example.
Conventional discharge-pumped gas lasers with preionizers have a preionizing circuit included in a main discharge circuit so that the preionizer electrodes are directly and simultaneously driven by the main discharge circuit. When a peaking capacitor of the main discharge circuit is charged, the current also flows through the preionizer electrodes, and when a main discharge is produced between main discharge electrodes the discharging current also flows through the preionizer electrodes. Therefore, an unduly large current, larger than necessary to preionize the laser gas between the main discharge electrodes flows through the preionizer electrodes, producing a spark discharge that is more intensive than necessary. The preionizer electrodes thus cause an unwanted energy loss, resulting in a reduction in laser emission efficiency. The preionizer electrodes are also liable to get worn rapidly and become unstable in operation after long usage. The preionizer electrodes produce a relatively large amount of minute particles, which are deposited on optical windows, thus lowering the intensity of laser emission.
Since the preionizer electrodes are included in the main discharge circuit, the main discharge circuit is relatively large in size, and hence cannot produce a main discharge at high speed. With the slow main discharge, the main discharge uniformity and the laser emission efficiency are low.